1. Field of the Invention
The present invention relates to a light source module, an optical scanning device, and an image forming apparatus.
2. Description of the Related Art
An optical scanning device that optically scans a scanning surface such that an optical beam emitted from a laser light source is deflected by a deflecting unit and the deflected optical beam is focused on the scanning surface as a beam spot by an imaging optical system has been widely used in conjunction with an image forming apparatus, such as a digital copier, a laser printer, a laser facsimile machine, and a laser plotter. There has been developed various types of optical scanning devices.
In technologies related to an optical scanning, one of issues that have been consistently expected to achieve is “speeding up of the optical scanning”. An image formation by the optical scanning is performed in such a manner that a photosensitive surface of a photosensitive element is exposed to a beam spot to which a laser beam is focused by each 1-dot. An optical energy required for the 1-dot exposure is obtained by the product of a 1-dot exposure time and a light intensity of the beam spot. To realize the “speeding up of the optical scanning”, the 1-dot exposure time needs to be reduced, i.e., it is essential to increase the “light intensity of the beam spot”.
As one of methods for a high-speed image formation by an optical scanning, there is a multibeam optical scanning. In this method, optical writings on a plurality of scanning lines can be performed at the same time by a one-time optical scanning, so that an image forming speed can be dramatically improved. However, even in a case of the multibeam optical scanning, there has still been expected to shorten a scanning time, so that it is necessary to increase a light intensity of a beam spot.
Furthermore, a vertical cavity surface emitting laser (VCSEL) has been put to practical use in recent years. The VCSEL is suited to a light source for the multibeam optical scanning because a plurality of light-emitting sources can be easily arrayed on the same plane. However, an emission intensity of the VCSEL is lower than that of a conventionally-known edge emitting laser diode. Therefore, in this case also, it is a matter of how to increase a light intensity of a beam spot.
A light intensity of a beam spot is determined depending on an emission intensity of a light source and a propagating efficiency of an optical beam from the light source down to a scanning surface. Therefore, to increase the light intensity of the beam spot, it is also necessary to increase the propagating efficiency of the optical beam.
In an optical scanning device, when a propagating efficiency of an optical beam decreases drastically, as one of factors, it can be considered that the optical beam is shielded by “an apertured plate for beam shaping”. As is commonly known, when an optical beam emitted from a laser light source is focused as a beam spot, a beam spot size is inversely proportional to a numerical aperture of a lens that condenses the optical beam to the beam spot, and is proportional to a wavelength. Therefore, as an aperture size (diameter) of the apertured plate increases, the beam spot size is getting decreased.
A size of 1 dot in an image formation is determined depending on the beam spot size, and also determined based on a specification of the optical scanning device in design. If the beam spot size is excessively larger than a designed value, it is not possible to meet “a specification for a resolution of an image to be formed” that is required for the optical scanning device, and thereby causing a decrease in an image quality of the formed image. On the contrary, if the beam spot size is excessively smaller than the designed value, a space is generated between adjacent dots, and thereby causing a decrease in an image quality of the formed image.
To prevent such a situation, “the beam spot size” needs to be “within an allowable range” around the designed value.
The beam spot size denotes “a diameter of a beam waist” formed on a portion of the focused optical beam in design. The beam size (diameter) is getting larger with increasing a distance from a position of the beam waist.
The optical scanning device is designed so that the position of the beam waist conforms to a scanning surface. However, in an actually-manufactured optical scanning device, it is not possible to avoid a manufacturing error or an assembling error of components and optical elements. Therefore, a “misalignment” generally occurs due to a positional error between the position of the beam waist and “a surface of a photosensitive image carrier as the scanning surface”. When the scanning surface does not conform to the position of the beam waist due to the “misalignment”, the beam spot size on the scanning surface increases when the misalignment occurs in either direction, i.e., whether in a travel direction of the optical beam or not. Such “an increase in the beam spot size due to the misalignment” is called “a spot size dilution”, and the misalignment is called “a defocus”.
An allowable range of the “the spot size dilution” with respect to the beam spot size in design is called “a depth allowance”. The depth allowance is “a range of the defocus in a direction of an optical axis to stay within an allowable beam spot size (for example, to stay within a range of the beam spot size to 10% increase of which)”. When the defocus of the optical beam with respect to the scanning surface is within the depth allowance, an actual beam spot size on the scanning surface is within the allowable range of the spot size dilution, so that the optical scanning can be performed appropriately.
There is a limit to reduce an error in a manufactured optical scanning device, so that the depth allowance is preferably made as large as possible.
To increase a light intensity of a beam spot by improving “a propagating efficiency of an optical beam” in an optical scanning, there is a way to “decrease a light shielding rate of the apertured plate (i.e., increase an amount of light passing through an aperture of the apertured plate) by increasing an aperture size of the aperture. However, when the aperture size is increased to a degree effective in increasing the light intensity, a diameter of the beam waist is reduced, whereby the beam spot size on the scanning surface is reduced beyond “the allowable range of the beam spot size”. Consequently, a state of convergence of the optical beam toward the beam waist becomes precipitous, resulting in a reduction of “the depth allowance”. Such a reduction of the beam waist size caused by the increase in the aperture size of the aperture will be referred to as “a spot size concentration”.
Therefore, it is not preferable to “simply increase the aperture size of the aperture” to increase the light intensity of the beam spot because it causes such a side-effect as “the spot size concentration or narrowing of the depth allowance”.
As a method for preventing a reduction of a beam spot size or a reduction of a depth allowance while increasing an amount of light passing through an apertured plate by increasing an aperture size of an aperture of the apertured plate, Japanese Patent Application Laid-open No. 2006-234955 and Japanese Patent Application Laid-open No. 2006-234956 disclose “a method of superimposing optical beams branched by a diffraction grating on a scanning surface”. However, in this method, it is necessary to perform a phase focusing among a plurality of the branched optical beams at high accuracy. Therefore, it is difficult to obtain desired characteristics due to a production tolerance or a temporal change.
Furthermore, Japanese Patent Application Laid-open No. 2006-234955 and Japanese Patent Application Laid-open No. 2006-234956 discloses “a method of superimposing optical beams on an imaging surface by changing a state of deflection of a part of the optical beams”. However, in this method, the optical beams in different deflection states from one another are incoherently superimposed (superimposed based on the intensity), so that the optical beams tend to spread, and thus it is difficult to ensure a depth allowance.